Pablo Jarillo-Herrero

Harvard University, Cambridge, Massachusetts, United States

Are you Pablo Jarillo-Herrero?

Claim your profile

Publications (115)1115.86 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Ultrafast electron thermalization - the process leading to Auger recombination, carrier multiplication via impact ionization and hot carrier luminescence - occurs when optically excited electrons in a material undergo rapid electron-electron scattering to redistribute excess energy and reach electronic thermal equilibrium. Due to extremely short time and length scales, the measurement and manipulation of electron thermalization in nanoscale devices remains challenging even with the most advanced ultrafast laser techniques. Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials in order to introduce a highly tunable electron transfer pathway that directly competes with electron thermalization. We realize this scheme in a graphene-boron nitride-graphene (G-BN-G) vdW heterostructure, through which optically excited carriers are transported from one graphene layer to the other. By applying an interlayer bias voltage or varying the excitation photon energy, interlayer carrier transport can be controlled to occur faster or slower than the intralayer scattering events, thus effectively tuning the electron thermalization pathways in graphene. Our findings, which demonstrate a novel means to probe and directly modulate electron energy transport in nanoscale materials, represent an important step toward designing and implementing novel optoelectronic and energy-harvesting devices with tailored microscopic properties.
    No preview · Article · Jan 2016 · Nature Physics
  • Source
    Article: Retraction
    Y.H. Wang · J.R. Kirtley · F. Katmis · P. Jarillo-Herrero · J.S. Moodera · K.A. Moler

    Preview · Article · Dec 2015 · Science
  • [Show abstract] [Hide abstract]
    ABSTRACT: Diverse parallel stitched two-dimensional heterostructures are synthesized, including metal-semiconductor (graphene-MoS2), semiconductor-semiconductor (WS2-MoS2), and insulator-semiconductor (hBN-MoS2), directly through selective sowing of aromatic molecules as the seeds in chemical vapor deposition (CVD) method. Our methodology enables the large-scale fabrication of lateral heterostructures with arbitrary patterns, and clean and precisely aligned interfaces, which offers tremendous potential for its application in integrated circuits.
    No preview · Article · Dec 2015 · Advanced Materials
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Scanning SQUID is a local magnetometer which can image flux through its pickup loop due to DC magnetic fields ($\Phi$). Scanning SQUID can also measure a sample's magnetic response to an applied current ($d\Phi/dI$) or voltage ($d\Phi/dV$) using standard lock-in techniques. In this manuscript, we demonstrate that electric coupling between the scanning SQUID and a back gate-tuned, magnetic sample can lead to a gate-voltage dependent artifact when imaging $d\Phi/dI$ or $d\Phi/dV$. The electric coupling artifact results in $d\Phi/dV$ and $d\Phi/dI$ images which mimic the spatial variation of the static magnetic fields from the sample (e.g. ferromagnetic domains). In back-gated $EuS/Bi_2Se_3$ bilayers, we show that the electric coupling effect is important, and is responsible for the reported signal from chiral currents in Y.H. Wang, et al. (DOI: 10.1126/science.aaa0508). Previous scanning SQUID current imaging experiments are unaffected by this artifact, as they are either on non-magnetic samples or the spatial distribution of magnetism does not match the features observed in $d\Phi/dI$. In conclusion, $d\Phi/dI$ or $d\Phi/dV$ imaging of magnetic, back-gated samples should only be applied and interpreted with great caution.
    Preview · Article · Dec 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this work, we leverage graphene's unique tunable Seebeck coefficient for the demonstration of a graphene-based thermal imaging system. By integrating graphene based photo-thermo-electric detectors with micro-machined silicon nitride membranes, we are able to achieve room temperature responsivities on the order of ~7-9 V/W (at λ=10.6 μm), with a time constant of ~23 ms. The large responsivities, due to the combination of thermal isolation and broadband infrared absorption from the underlying SiN membrane, have enabled detection as well as stand-off imaging of an incoherent blackbody target (300-500K). By comparing the fundamental achievable performance of these graphene-based thermopiles with standard thermocouple materials, we find extrapolate that graphene's high carrier mobility can enable improved performances with respect to two main figures of merit for infrared detectors: detectivity (>8x10(8) cmHz(1/2)W(-1)) and noise equivalent temperature difference (<100 mK). Furthermore, even average graphene carrier mobility (<1000 cm(2)/Vs) is still sufficient to detect the emitted thermal radiation from a human target.
    No preview · Article · Oct 2015 · Nano Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Opto-electronic devices utilizing graphene have already demonstrated unique capabilities, which are much more difficult to realize with conventional technologies. However, the requirements in terms of material quality and uniformity are very demanding. A major roadblock towards high-performance devices are the nanoscale variations of graphene properties, which strongly impact the macroscopic device behaviour. Here, we present and apply opto-electronic nanoscopy to measure locally both the optical and electronic properties of graphene devices. This is achieved by combining scanning near-field infrared nanoscopy with electrical device read-out, allowing infrared photocurrent mapping at length scales of tens of nanometers. We apply this technique to study the impact of edges and grain boundaries on spatial carrier density profiles and local thermoelectric properties. Moreover, we show that the technique can also be applied to encapsulated graphene/hexagonal boron nitride (h-BN) devices, where we observe strong charge build-up near the edges, and also address a device solution to this problem. The technique enables nanoscale characterization for a broad range of common graphene devices without the need of special device architectures or invasive graphene treatment.
    Full-text · Article · Sep 2015
  • Source
    Y. H. Wang · J. R. Kirtley · F. Katmis · P. Jarillo-Herrero · J. S. Moodera · K. A. Moler
    [Show abstract] [Hide abstract]
    ABSTRACT: A magnetic domain boundary on the surface of a three-dimensional topological insulator is predicted to host a chiral edge state, but direct demonstration is challenging. We used a scanning superconducting quantum interference device to show that current in a magnetized topological insulator heterostructure (EuS/Bi2Se3) flows at the edge when the Fermi level is gate-tuned to the surface band gap. We further induced micrometer-scale magnetic structures on the heterostructure and detected a chiral edge current at the magnetic domain boundary. The chirality of the current was determined by magnetization of the surrounding domain, and its magnitude by the local chemical potential rather than the applied current. Such magnetic structures provide a platform for detecting topological magnetoelectric effects and may enable progress in quantum information processing and spintronics.
    Preview · Article · Aug 2015 · Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hexagonal boron nitride (h-BN) is a natural hyperbolic material 1 , in which the dielectric constants are the same in the basal plane (ε t ≡ ε x = ε y) but have opposite signs (ε t ε z < 0) in the normal plane (ε z) 1–4. Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons 1,2,5 —collective modes that originate from the coupling between photons and electric dipoles 6 in phonons. However, control of these hyperbolic phonon polari-tons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN 1,2,7. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure 8 composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene 9–13 with hyperbolic phonon polaritons in h-BN 1,2 , so that the eigenmodes of the gra-phene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in gra-phene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial 14 as the resulting properties of these devices are not present in its constituent elements alone. Van der Waals (vdW) heterostructures assembled from mono-layers (one or a few) of graphene, hexagonal boron nitride (h-BN), MoS 2 and other atomic crystals in various combinations are emerging as a new paradigm with which to attain desired electronic 8,15
    Full-text · Article · Jun 2015 · Nature Nanotechnology
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Interference of standing waves in electromagnetic resonators forms the basis of many technologies, from telecommunications and spectroscopy to detection of gravitational waves. However, unlike the confinement of light waves in vacuum, the interference of electronic waves in solids is complicated by boundary properties of the crystal, notably leading to electron guiding by atomic-scale potentials at the edges. Understanding the microscopic role of boundaries on coherent wave interference is an unresolved question due to the challenge of detecting charge flow with submicron resolution. Here we employ Fraunhofer interferometry to achieve real-space imaging of cavity modes in a graphene Fabry-Perot resonator, embedded between two superconductors to form a Josephson junction. By directly visualizing current flow using Fourier methods, our measurements reveal surprising redistribution of current on and off resonance. These findings provide direct evidence of separate interference conditions for edge and bulk currents and reveal the ballistic nature of guided edge states. Beyond equilibrium, our measurements show strong modulation of the multiple Andreev reflection amplitude on an off resonance, a direct measure of the gate-tunable change of cavity transparency. These results demonstrate that, contrary to the common belief, electron interactions with realistic disordered edges facilitate electron wave interference and ballistic transport.
    Full-text · Article · Jun 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Hybrid graphene-topological insulator (TI) devices were fabricated using a mechanical transfer method and studied via electronic transport. Devices consisting of bilayer graphene (BLG) under the TI Bi$_2$Se$_3$ exhibit differential conductance characteristics which appear to be dominated by tunneling, roughly reproducing the Bi$_2$Se$_3$ density of states. Similar results were obtained for BLG on top of Bi$_2$Se$_3$, with 10-fold greater conductance consistent with a larger contact area due to better surface conformity. The devices further show evidence of inelastic phonon-assisted tunneling processes involving both Bi$_2$Se$_3$ and graphene phonons. These processes favor phonons which compensate for momentum mismatch between the TI $\Gamma$ and graphene $K, K'$ points. Finally, the utility of these tunnel junctions is demonstrated on a density-tunable BLG device, where the charge-neutrality point is traced along the energy-density trajectory. This trajectory is used as a measure of the ground-state density of states.
    Preview · Article · Apr 2015 · Physical Review B
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A far-reaching goal of graphene research is exploiting the unique properties of carriers to realize extreme nonclassical electronic transport. Of particular interest is harnessing wavelike carriers to guide and direct them on submicron scales, similar to light in optical fibers. Such modes, while long anticipated, have never been demonstrated experimentally. In order to explore this behavior, we employ superconducting interferometry in a graphene Josephson junction to reconstruct the real-space supercurrent density using Fourier methods. Our measurements reveal charge flow guided along crystal boundaries close to charge neutrality. We interpret the observed edge currents in terms of guided-wave states, confined to the edge by band bending and transmitted as plane waves. As a direct analog of refraction-based confinement of light in optical fibers, such nonclassical states afford new means for information transduction and processing at the nanoscale.
    Full-text · Article · Apr 2015 · Nature Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials light propagation is unusual, leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride (hBN), a natural mid-infrared hyperbolic material, can act as a "hyper-focusing lens" and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon-polaritons launched by metallic disks underneath the hBN crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial hyperbolic materials with lower losses and higher optical localization.
    Full-text · Article · Apr 2015 · Nature Communications
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications.
    Full-text · Article · Apr 2015 · Nature Nanotechnology
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report high quality graphene and WSe2 devices encapsulated between two hexagonal boron nitride (hBN) flakes using a pick-up method with etched hBN flakes. Picking up pre-patterned hBN flakes to be used as a gate dielectric or mask for other 2D materials opens new possibilities for the design and fabrication of 2D heterostructures. In this letter, we demonstrate this technique in two ways: first, a dual-gated graphene device that is encapsulated between an hBN substrate and pre-patterned hBN strips. The conductance of the graphene device shows pronounced Fabry-Pérot oscillations as a function of carrier density, which implies strong quantum confinement and ballistic transport in the locally gated region. Second, we describe a WSe2 device encapsulated in hBN, with the top hBN patterned as a mask for the channel of a Hall bar. Ionic liquid selectively tunes the carrier density of the contact region of the device, while the hBN mask allows independent tunability of the contact region for low contact resistance. Hall mobility larger than 600 cm(2)/(V·s) for few-layer p-type WSe2 at 220 K is measured, the highest mobility of a thin WSe2 device reported to date. The observations of ballistic transport in graphene and high mobility in WSe2 confirm pick-up of pre-patterned hBN as a versatile technique to fabricate ultra-clean devices with high quality contact.
    No preview · Article · Feb 2015 · Nano Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have implemented and investigated the tunable hyperbolic response in heterostructures comprised of a monolayer graphene deposited on hexagonal boron nitride (G-hBN) slabs. Electrostatic gating of the graphene layer enables electronic tunability of phonon polaritonic properties of hBN: a layered material with well-documented hyperbolic response in the mid-infrared (mid-IR) frequencies. The tunability originates from the hybridization of surface plasmon polaritons in graphene to hyperbolic phonon polaritons in hBN: an effect that we examined via nano-IR imaging and spectroscopy. The hybrid polaritons possess combined virtues from plasmons in graphene and phonon polaritons in hBN. Therefore, G-hBN structures fulfill the definition of the electromagnetic metamaterial since the attained property of these devices is not revealed by its constituent elements. Our results uncover a practical approach for realization of agile nano-photonic metamaterials by exploiting the interaction of distinct types of polaritonic modes hosted by different constituent layers of van der Waals heterostructures.
    Full-text · Article · Jan 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
    Full-text · Article · Nov 2014 · Journal of Physics Condensed Matter
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Controlling the energy flow processes and the associated energy relaxation rates of a light emitter is of high fundamental interest, and has many applications in the fields of quantum optics, photovoltaics, photodetection, biosensing and light emission. While advanced dielectric and metallic systems have been developed to tailor the interaction between an emitter and its environment, active control of the energy flow has remained challenging. Here, we demonstrate in-situ electrical control of the relaxation pathways of excited erbium ions, which emit light at the technologically relevant telecommunication wavelength of 1.5 $\mu$m. By placing the erbium at a few nanometres distance from graphene, we modify the relaxation rate by more than a factor of three, and control whether the emitter decays into either electron-hole pairs, emitted photons or graphene near-infrared plasmons, confined to $<$15 nm to the sheet. These capabilities to dictate optical energy transfer processes through electrical control of the local density of optical states constitute a new paradigm for active (quantum) photonics.
    Full-text · Article · Oct 2014 · Nature Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $\mu$(n), which is found to be consistent with independent ARPES measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.
    Full-text · Article · Oct 2014 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV.
    Preview · Article · Jun 2014 · APL Materials
  • Source
    Hugh Olen Hill Churchill · Pablo Jarillo-Herrero
    [Show abstract] [Hide abstract]
    ABSTRACT: Graphene was first isolated by exfoliating single layers from a graphite crystal using Scotch tape. This method was later applied to other materials with layered structures, creating a family of atomically layered materials that includes insulators such as hexagonal boron nitride, metals such as NbSe[subscript 2], and semiconductors such as MoS[subscript 2] and WSe[subscript 2]. All of these materials had been studied for decades in bulk form, but their exfoliated, two-dimensional form gave them new life and properties. Writing in Nature Nanotechnology, Xian Hui Chen, Yuanbo Zhang and co-workers have now similarly brought black phosphorus back to the spotlight, which is the most stable and least reactive form of elemental phosphorus, and was discovered in bulk form 100 years ago.
    Preview · Article · May 2014 · Nature Nanotechnology

Publication Stats

5k Citations
1,115.86 Total Impact Points

Institutions

  • 2015
    • Harvard University
      Cambridge, Massachusetts, United States
  • 2009-2015
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2007
    • Columbia University
      • Department of Physics
      New York City, New York, United States
  • 2004-2007
    • Delft University of Technology
      • Applied Geophysics and Petrophysics
      Delft, South Holland, Netherlands